Metal roof structures typically comprise a series of parallel rafter beams extending across the building in one direction and purlin beams parallel to each other mounted on top of the rafters extending in a direction normal to the rafters. The purlins are often joined together by support bracing or straps for extra support. Insulation material in long sheets is placed in the area between purlins. The sheets of insulation material can be laid along the length of the purlins or across the purlins in a direction normal to the purlins. If desired, the roof structure can have a first layer of insulation material which is laid along the length of the purlins, and a second layer of insulation material which is laid laterally across the purlins on top of the first layer on insulation. Hard roofing material such as metal decking is then attached on top of the purlins over the insulation material. Because the hard roofing material comes in long sheets and the roofs generally have two sloped sections, it is customary to construct the roof along the length of the structure from one end to the other. The workers stand on the previously laid section of roof to construct the next section.
The insulation material must be supported between the purlins beneath the hard roofing material. Various methods of supporting the insulation material have been used. Mounting straps or wire mesh which are attached to or draped over the purlins forming a lattice have been used. This is referred to as banding. A sheet, typically made of vinyl and acting as a vapor barrier, is then rolled onto the lattice, and insulation material is placed between adjacent purlins and over the sheet. If the installation of the lattice is done from underneath the roof structure, scaffolding or lifting equipment is typically required for installation. Since the lattice encompasses the entire roof, installation is costly and time consuming. Once the hard roofing material is mounted on the purlins, the sheet can support the insulation material and the lattice no longer serves any useful purpose.
Some systems dispense with the lattice and use the sheet itself to support the insulation material. The support sheet is draped from the adjacent purlins and the insulation material is placed on top of the support sheet. A carriage is typically used to aid in the dispensing of the support sheet wound on a roll. The carriage is positioned on top of the purlins and travels the length of the purlins during the roof construction. As the carriage travels the length of the purlins, the support sheet is draped across the purlins.
A safety netting which is attached to the purlins and rafters is often used to help prevent workers from accidentally falling through the open spaces of the roof structure. The netting is typically laid over the tops of the purlins and is covered over by the support sheet as the roof is constructed.
The carriages have been equipped with various structures which ride on top of the purlins and guide the carriage as it travels the length of the purlins. For example, a pair of beams having an L-shaped cross-section mounted on the bottom of the carriage and above two adjacent purlins have been used. The L-shaped beams are typically rigidly fixed to the carriage. The L-shaped beams have a laterally extending leg which slides on the top surface of the purlins, and a downwardly extending leg which contacts the side of the purlin, thus laterally guiding the carriage. The L-shaped beams, however, cannot be used with the safety netting because the netting will get caught between the beam and the top of the purlin. Also, because the purlin support bracing is generally attached towards the top portion of the purlins, the downwardly extending leg of the L-shaped beam often hits the support bracing. Also, bolts or rivets which extend up above the tops of the purlins may obstruct parts of the beam, such as the laterally extending leg of the L-shaped beam. In these cases, the carriage must be lifted up over the obstruction so that the carriage can continue moving along the length of the purlins.
Some prior art carriages are equipped with cylindrical rollers which roll along the top portions of the purlins. To prevent the carriage from moving laterally along the length of the purlins, the carriages are equipped with guides to maintain the carriage in alignment with the purlins. Typical guides used in the industry extend downwardly and are positioned adjacent the purlins, thereby contacting the side of the purlin to prevent the carriage from shifting laterally as it rolls on top of the purlins. These downwardly extending guides are also obstructed by the bracing connecting adjacent purlins. Although some guides have the ability to deflect out of the way either by manual operation or by automatic means, the guides often tear the safety netting if one is used on the roof. Also, the spacing of the purlins is not always accurate and the guides can jam the carriage between the purlins if the spacing of the purlins is shorter than specified.